Add note about using graphical services to README

README.md

@@ -161,6 +161,40 @@ Existing file '/home/jdoe/.gitconfig' is in the way
 Please move the above files and try again
 ```
 
+Graphical services
+------------------
+
+Home Manager includes a number of services intended to run in a
+graphical session, for example `xscreensaver` and `dunst`.
+Unfortunately, such services will not be started automatically unless
+you let Home Manager start your X session. That is, you have something
+like
+
+```nix
+{
+  # …
+
+  services.xserver.enable = true;
+
+  # …
+}
+```
+
+in your system configuration and
+
+```nix
+{
+  # …
+
+  xsession.enable = true;
+  xsession.windowManager = "…";
+
+  # …
+}
+```
+
+in your Home Manager configuration.
+
 [Nix]: https://nixos.org/nix/
 [NixOS]: https://nixos.org/
 [Nixpkgs]: https://nixos.org/nixpkgs/